In metallurgy, the temperature of molten steel needs to be measured continuously. The steel making process is essentially the process of controlling the temperature and the ingredients of the molten steel, so measuring the temperature of the molten steel continuously is significant for improving productive efficiency, decreasing energy consumption and improving quality. The conventional technique for the measurement works in an object-contacted way. A platinum-rhodium thermo-couple with a protector is put into the molten steel to sense the temperature. However, the protector's lifetime is limited because of the erosion of steel residue and the thermo-couple is expensive, so the cost of measurement is high, which is difficult to be accepted by steel making plants.
U.S. Pat. No. 3,432,522 discloses a device of continuously measuring the temperature of molten steel, utilizing the blackbody cavity theory. According to this patent, a tube is put into the molten steel and the temperature of the molten steel is obtained from the radiation emitted by the tube and received by an infrared detector. However, many disadvantages exist in this patent. Firstly, blackbody cavity is a special physical concept and physical model generating stable spectral irradiation similar with that of the perfect blackbody. The radiant traits of a cavity depend on the structure, the material and other factors of the cavity, therefore, not every cavity can be called as a blackbody cavity. This patent fails to describe the structure of the tube in detail and teach how deeply the tube should be put into the molten steel. Moreover, whether the ceramic portion of the tube should immerge under the surface of the molten steel is not mentioned. Therefore, the tube in this patent should not be called as a blackbody cavity. Consequently, errors will occur in the results of the measurement. Secondly, graphite used at the end of the tube is of strong volatility under high temperature environment, and the volatile gas can pollute the optical path and significantly affect the accuracy of the measurement.